The skin of human and other animal bodies comprises the epidermis and dermis. The epidermis is outermost and comprises a number of layers or strata. The epidermis is continually renewed, through production of cells in the innermost stratum, which cells move outwards until the outermost stratum of the epidermis, the stratum corneum, is reached. During this process, the cells die, their contents are substantially replaced by keratin, and their shape changes from substantially spheroid to a more flattened shape. The stratum corneum is therefore made up of dead cells. These are provided in a number of layers, for example 10 to 150 layers. Cells in each layer, because of their flattened shape, can overlap with cells in adjacent layers forming a barrier for the body which has low permeability. The barrier is important for protecting the body from loss of water and the ingress of harmful substances. The barrier, however, also substantially impedes the penetration of beneficial materials placed on the skin and therefore reduces the ability to deliver the materials to deeper layers of the skin and other parts of the body. On their own, beneficial materials often cannot penetrate the barrier of the skin provided by the layers of the stratum corneum quickly and efficiently enough to provide sufficient therapeutic effects.
Various techniques have been developed for mitigating the low permeability of the stratum corneum barrier, to allow therapeutic materials past the barrier using minimally invasive technologies that can penetrate the skin's outermost layers without stimulating the nerves. Such techniques include chemical diffusion enhancers, iontophoretic devices, adhesives, microneedle arrays, and sonophoretic devices. While many of these techniques can be effective, their use can be hampered by side effects and often also require application by professional personnel.